In order to record information on an optical information recording medium, which is typically an optical disc, it is necessary to optimally adjust the emission power of a laser with respect to an information recording surface of the medium. In general, the characteristics of a semiconductor laser, for example, widely varies due to a change in the environmental temperature or the deterioration of the laser itself. Therefore, a control means which is capable of outputting an appropriate power for recording of information on an optical information recording medium according to such characteristic variation is required.
Hereinafter, characteristics of a laser are briefly described by using a semiconductor laser as an example. In the discussion below, the semiconductor laser is used as an example of such a laser.
FIG. 19 shows an I-L characteristic (Injection current—Light intensity characteristic) for temperatures T1 and T2. When driven with a current which is greater than a threshold current Ith, oscillation of the laser begins. Within an oscillation range, the light output per unit driving current increases relative to quantum efficiency η. In FIG. 19, threshold current Ith0 and quantum efficiency η0 for temperature T1, and threshold current Ith1 and quantum efficiency η1 for temperature T2 are shown. The threshold current Ith and quantum efficiency η change according to a variation in the environmental temperature. Such changes are different among lasers. There is a laser where changes of threshold current Ith and quantum efficiency η for temperature T1 are twice as much as those for temperature T2, as shown in an example illustrated in FIG. 19. Thus, the power of an optical output to be emitted is widely changed according to environmental conditions even when a driving current of the laser is the same. Therefore, laser power control is typically achieved by changing a driving current such that the power of an optical output is set to a desired power.
Next, a conventional laser power control method is described with reference to FIG. 20. FIG. 20 illustrates a method for controlling the power of an optical output emitted by a laser so as to be a power suitable for recording on an information recording surface of a medium while monitoring light reflected from the medium.
In FIG. 20, an optical beam output from a laser 201 is reflected by a medium and then received by a light receiving element 202. The light receiving element 202 converts the power of the received optical output to an electric signal, which is output to a calculation section 203. The calculation section 203 compares a reference value stored therein with the output of the light receiving element 202 and calculates, based on the difference therebetween, a driving current where a variation of the laser power is corrected. The calculation section 203 then outputs the calculated driving current to a driving section 204. The driving section 204 drives the laser 201 based on the output of the calculation section 203. As an I-L characteristic of the laser 201 is varied, the power of the optical output emitted by the laser 201 changes. Accordingly, the power of light received by the light receiving element 202 after reflected by the medium also changes. Thus, the output value of the light receiving element 202 changes with respect to the reference value in the calculation section 203. The calculation section 203 adjusts a driving current of the driving section 204 based on a difference between the output value of the light receiving element 202 and the reference value. Due to controlled driving based on the adjusted driving current, the output power of light emitted by the laser 201 is controlled so as to be an appropriate power for recording on the medium.
When dirt, such as dust, fingerprint, etc., is present on the medium, the power of an optical output emitted by the laser 201 is partially scattered or absorbed due to such dirt before the optical output reaches an information recording surface of the medium. Thus, the power of the optical output is insufficient for an appropriate power for recording on the information recording surface of the medium. The power of light reflected by the medium is also insufficient in comparison to a case with no fingerprint or dirt present. The calculation section 203 compares the reference value stored therein with the output of the light receiving element 202 and calculates, based on the difference therebetween, a driving current by which the above insufficiency caused due to dust or dirt is compensated for, and then outputs the calculated driving current to the driving section 204. As a result, the output power of light emitted by the laser is increased. The output power of light is controlled such that the power of light reaching the information recording surface becomes an appropriate power for recording on the medium even when a portion of the power is partially scattered or absorbed by a fingerprint or dirt.
A representative example to which this technique is applied is ROPC (Running Optimum Power Control), which is used when recording information on a write once type optical disc, such as a CD-R or the like. The principles and means of ROPC are described in detail in CD-WO System Description Version 2.0.
The above conventional laser power control means, where an optical output is controlled so as to be at a power suitable for recording information on a medium while monitoring light reflected from a medium (or transmitted light), operates such that a correction is performed considering a power lost due to dust, a fingerprint, etc., with respect to a power optimum on an information recording surface, and an emission power varied due to a change of the I-L characteristic which is caused by a variation in the environmental temperature. That is, in the conventional method, it is not distinguishable whether a variation component of reflected light detected by the light receiving element 202 is a variation component resulting from a change in the I-L characteristic caused by a variation in the environmental temperature or a variation component generated by a fingerprint, or the like, present on the medium. Thus, according to the conventional method, when the power of reflected light is changed due to a change in the I-L characteristic which is caused by a variation in the environmental temperature, and there are a plurality of power values of an optical output which are required for recording information, the plurality of power values cannot be correctly controlled with a small error.
Now, this problem is described more specifically while referring to an example of recording information on a DVD-RAM optical disc.
In a DVD-RAM optical disc, the power of an optical output emitted by a laser used for forming a recording mark has three levels. That is, it is necessary to control the three powers during recording of information on the DVD-RAM optical disc. The three powers are Peak Power, Bias Power 1, and Bias Power 2 which are specified in the DVD Specifications for Rewritable Disk Version 1.0. In the descriptions below, the three powers are represented by abbreviations, Pk, Pb1, and Pb2, respectively.
FIG. 21 shows an example of the three powers emitted by a laser for forming a recording mark. FIG. 21(a) is a concept diagram wherein a period when a recording mark is formed is denoted by two levels, HIGH (H) and LOW (L), and the recording mark is formed on the medium during a HIGH (H) period. FIG. 21(b) is a concept diagram which illustrates the power of an optical output emitted by the laser during the HIGH (H) period shown in FIG. 21(a).
FIG. 21(b) shows an example where the three powers, Pk(=11 mW), Pb1(=5 mW), Pb2(=1 mW), are switched at a high speed (switching between Pk and Pb2 is achieved in about 34 ns), while using a reference power Pbase (=0 mW) indicated by a broken line. Although it is necessary to use a high-band receiving element in order to correctly receive the three powers of an optical output reflected by the medium which change at a high speed, such a high-band receiving element is very expensive and accordingly increases the cost of the apparatus. Thus, it is better to use a low-band receiving element in order to suppress an increase in the cost. In this case, the three powers of the optical output reflected by the medium, which are included in the power detected by the receiving element, cannot be divided into each of the three powers.
FIG. 22 shows a variation of a driving current which is necessary for outputting the three powers when the temperature changes from T1 to T2. In FIG. 22, Ith0 denotes a threshold current for temperature T1, and η0 denotes a quantum efficiency for temperature T1. Ith1 denotes a threshold current for temperature T2, and η1 denotes a quantum efficiency for temperature T2. Pk, Pb1, and Pb2 denote three powers used for recording information on a DVD-RAM disc. Ipk0 denotes a driving current which is necessary to output power Pk when the temperature is T1, Ibs10 denotes a driving current which is necessary to output power Pb1 when the temperature is T1, and Ibs20 denotes a driving current which is necessary to output power Pb2 when the temperature is T1. Ipk1 denotes a driving current which is necessary to output power Pk when the temperature is T2, Ibs11 denotes a driving current which is necessary to output power Pb1 when the temperature is T2, and Ibs21 denotes a driving current which is necessary to output power Pb2 when the temperature is T2. ΔIpk, ΔIbs1, and ΔIbs2 each denote a variation value of the driving current which is used for correcting each power value when the temperature is changed from T1 to T2.
From FIG. 22, the following relationships can be understood:ΔIpk=Ipk1−Ipk0=(Pk/η1+Ith1)−(Pk/η0+Ith0)  Expression (1)ΔIbs1=Ibs11−Ibs10=(Pb1/η1+Ith1)−(Pb1/η0+Ith0)  Expression (2)ΔIbs2=Ibs21−Ibs20=(Pb2/η1+Ith1)−(Pb2/η0+Ith0)  Expression (3)In this example where the power value received by the receiving element for temperature T1 is P0, the power value received by the receiving element for temperature T2 is P1, and the variation rate of P1 to P0 (P1/P0) is α, the control means calculates, based on the reflected light, the currents ΔIpk, ΔIbs1, and ΔIbs2 for driving the laser such that a variation rate a obtained based on the variation values of the power is 1, i.e., such that a variation of the power is eliminated.
The following description is based on an assumption that each of ΔIpk, ΔIbs1, and ΔIbs2 can be obtained by using variation value Δ. First, the following definition is given for the variation rate of reflected light which is caused by a variation of Pk:Ipk1/Ipk0=(Pk/η1+Ith1)/(Pk/η0+Ith0)=αpk Based on expression (1), ΔIpk can be expressed as follows:ΔIpk=αpk×Ipk0−Ipk0  Expression (4)Similarly, based on expressions (2) and (3), using αbs1 (the variation of reflected light which is caused by a variation of Pb1) and Δbs2 (the variation of reflected light which is caused by a variation of Pb2), the following expressions can be obtained:ΔIbs1=αbs1×Ibs10−Ibs10  Expression (5)ΔIbs2=αbs2×Ibs20−Ibs20  Expression (6)Since Pk≠Pb1≠Pb2, the following expression is established:(Pk/η1+Ith1)/(Pk/η0+Ith0)≠(Pb1/η1+Ith1)/(Pb1/η0+Ith0)≠(Pb2/η1+Ith1)/(Pb2/η0+Ith0)Therefore,αpk≠αbs1≠αbs2≠α(a variation rate of reflected light received by the receiving element)  Expression (7)Even if Pk is obtained from the value of α, such a value of a α is inconsistent with the above-established assumption, because the following relationships can be obtained from expressions (4), (5), (6), and (7)ΔIpk=α×Ipk0−Ipk0ΔIbs1≠α×Ibs10−Ibs10ΔIbs2≠α×Ibs20−Ibs20,that is, ΔIbs1 and ΔIbs2 cannot be obtained by using the variation value α.
This means that the control means cannot calculate, based on the reflected light, a driving current which is used for correcting the plurality of powers using a variation value of reflected light detected by the receiving element. Thus, according to the conventional method, there are a plurality of powers to be controlled, and the plurality of powers cannot be controlled with a small error when the power of an optical output emitted by the laser is partially scattered or absorbed by dust, a fingerprint, or the like, and deviated from an appropriate power for recording information on the medium under the circumstance where the environmental temperature changes.
Furthermore, reflected light is sometimes varied regardless of an optimum power for recording information on the medium due to production non-uniformity in the width of recording tracks and in the edge which is generated during production of the medium. In such a case, the power is varied because in the conventional method a variation component is detected by the receiving element, and the driving current is adjusted based on the difference between the detected component and the reference value stored in the calculation section. Accordingly, an error is caused in the power which has been controlled before a variation of reflected light generated due to the aforementioned production non-uniformity is detected.
The present invention was conceived in view of the above circumstances. An objective of the present invention is to provide a laser power control method for controlling a plurality of laser powers that are required for recording so as to be appropriate powers for recording even when the I-L characteristic of the laser is changed due to a variation in the environmental temperature and the laser power reaching a recording surface of a medium is deviated from an appropriate power due to dust, a fingerprint, or the like, or even when light reflected from the medium is varied regardless of an appropriate power for recording, and to provide an optical disc apparatus for performing such control.